MEASUREMENTS

Differential Phase
Figure 63. Modulated Ramp test signal.
Figure 64. Vector display of TSG-271 phase-alternate
Modulated Ramp.
• DEFINITION
Differential phase distortion, sometimes referred to as "diff
phase", "dP" or "dc/>", is present if a signal's chrominance
phase is affected by luminance level. This phase distortion is a
result of a system's inability to uniformly process the highfrequency
chrominance information at all luminance levels.
The amount of differential phase distortion is expressed in
degrees of subcarrier phase. Since both positive and negative
(lead and lag) phase errors may occur in the same signal, it is
important to specify whether the peak-to-peak phase error or
peak deviation from the blanking level phase is being quoted.
PAL measurement standards most frequently refer to peak deviation
differential phase measurements. Two numbers are typically
given to describe the distortion: the peak positive phase deviation
and the peak negative phase deviation from the subcarrier
phase at blanking level. Sometimes the larger of these two
values is given as a single peak result.
This distortion should be measured at different APLs, and the
worst error should be quoted.
• PICTURE EFFECTS
Since virtually all PAL receivers now employ delay-line decoders,
reasonable amounts of differential phase distortion cannot be
readily detected in the picture. A delay-line decoder averages
each two successive lines in the field, and the resultant information
is displayed. Chrominance phase shifts are therefore
canceled out, and do not result in a hue shift in the picture.
(Differential phase is actually converted to differential gain in the
resultant, but gain errors are much less objectionable in the picture.)
• TEST SIGNALS
This distortion is measured with a test signal which consists of
uniform-phase chrominance superimposed on different luminance
levels. A Modulated Staircase (5 or 10 step) or a Modulated
Ramp (See Figure 63) is typically used. A ramp is normally
used when the signal is passing through digital systems.
Some generators, such as the Tektronix TSG-271, offer a phasealternate
Modulated Ramp test signal. A vector display of this
signal is shown in Figure 64. This signal can help you detect
distortions which have affected the U and V components
differently, which is most likely to occur if the signal has been
demodulated and the U and V components passed through
separate processing channels. If this signal is available, you
may wish to repeat the measurement procedures outlined
below for both signal vectors.
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